I've sent off the board and the first 3 planks for fabrication! If you wanted to submit one in the first batch and get it made for free (and a free Loki!) you've missed your chance, but I'll undoubtedly be doing more later.

1) Use of a RRIO OpAmp vs a LM358, to increase drive level to MOSFET. Makesure your choice can handle large C load, eg. stability reasons. Design wiseinsure power sequencing in this circuit cannot cause phase reversal of theOpAmp outputs. Stated another way make sure OpAmp power supply comesup before any other pin on the OpAmp.

2) For 3.3V Regulator, strap a diode across it to prevent blowing out reg ifits input is shorted to ground, caused by its output cap C18 discharging backthru regulator. That would be D1 in this example.

3) Some place on board edge for a large ground post where scope probe groundclips can be placed w/o shorting any other part of board. That would be atleast two or more scope ground lead clips/alligators.

4) Use polymer tanatalums in BOM for any bulk caps, they have ~ 10 x betterfreq response curves than regular tantalums, lower noise in board. Close tothe polymer a place for a SMT ceramic as well, .1 or .01 uF.

1) Use of a RRIO OpAmp vs a LM358, to increase drive level to MOSFET. Makesure your choice can handle large C load, eg. stability reasons. Design wiseinsure power sequencing in this circuit cannot cause phase reversal of theOpAmp outputs. Stated another way make sure OpAmp power supply comesup before any other pin on the OpAmp.

I based the design on that used in the Arduino and the Eleven. As long as its output is over the mosfet's threshold, though, it should work fine, no? I presume that it does, since it's widely used elsewhere.

2) For 3.3V Regulator, strap a diode across it to prevent blowing out reg ifits input is shorted to ground, caused by its output cap C18 discharging backthru regulator. That would be D1 in this example.

3 terminal regulator.jpg

Fair point. I'll fix that for the next revision.

3) Some place on board edge for a large ground post where scope probe groundclips can be placed w/o shorting any other part of board.

Also a fair point. I think the mounting holes ought to be shorted to ground, but a better place for this wouldn't hurt.

4) Use polymer tanatalums in BOM for any bulk caps, they have ~ 10 x betterfreq response curves than regular tantalums, lower noise in board.

For the switching regulator and the linear regulator, I've used the caps recommended in their datasheets and by the TI designer. For the decoupling caps on the MPU, I've used standard ceramics, but I don't think that's what you had in mind by bulk caps. I don't think that leaves any?

danadak wrote:2) For 3.3V Regulator, strap a diode across it to prevent blowing out reg ifits input is shorted to ground, caused by its output cap C18 discharging backthru regulator. )

If the 3.3 reg is an TLV702xx as indicated in the schematic there's no need for a protection diode since it's already included in the device. No extra protection is necessary unless unhealthy amount (hundreds of uF) of capacitance is connected to it's output.

danadak wrote:Use polymer tanatalums in BOM for any bulk caps, they have ~ 10 x betterfreq response curves than regular tantalums, lower noise in board. Close tothe polymer a place for a SMT ceramic as well, .1 or .01 uF.

Unless really necessary it might be a good idea to avoid tantalum caps. They are more expensive than ceramics, prone to spectacularly explode when reversed or even slightly overvoltaged, and they are using a conflict mineral. And many modern multilayer ceramics actually have a better ESR compared to tantalums - especially in the higher frequency regions. It seems like most vreg manufacturers recommend ceramics in their datasheets.

I used the skillet reflow method for the first time, with a solder paste stencil, and I'm glad I did. This is definitely the finest and trickiest job I've done by a fair margin. Lining up the stencil took some care, as did positioning the processor, with 0.4mm pitch. My USB microscope was invaluable for verifying placement.

Reflow was quick and painless. The electric skillet I got heats up very quickly. The IR thermometer I bought turned out to be more or less entirely surplus to requirements.

After reflow, the processor had quite a few bridges, which were (eventually!) fixed with a fine tipped soldering iron, lots of flux, and some solder wick. I think a stencil with smaller apertures is required.

The board works perfectly so far - I haven't identified any problems besides my buying the wrong footprint of DC jack, easily remedied. I've also started building up a display plank, but it'll be a while before I get a chance to write code to actually send data to the display.

For what it's worth, the offer of free PCB manufacturing for OSHW plank designs is still open, as is the offer of a Loki to anyone who promises to try it out and provide feedback.